Telephone system



July 25, 1950 l, MQLNAR 2,516,788

TELEPHONE SYSTEM Filed Nov. 21, 1945 v s sheets-sheet 1 FIG. 4

INCOMING TRU NK oFFacx-:A i T144 T414? SEL 2 a INDIVIDUAL LS.- TONE DETECTOR E 6 e@ '7 l Y INVENTOA IM R E MOLNAR ATTORNEY July 25, 1950 l. MOLNAR TELEPHONE SYSTEM s Sheets-sheet 2 Filed NOV. 2l, 1945 FIG.5.

1N VEN TOR. I M RE MOLNA R OFFICE B 0F Tous DETECIURS 7 i' COM GROUP OFFICE A A TTORN EY July 25, 1950 MoLNAR TELEPHONE SYSTEM 3 Sheets-Sheet 5 Filed Nov. 21, 1945 INVENTOR. IMRE MOLNAR T0 INGOMN; SEL EGTOR INCOMING TRUNK ATTOR N EY Patented July 25, 950

TELEPHONE SYSTEM Imre Molnar, Chicago, Ill., assignor to Automatic Electric Laboratories, Inc., Chicago, Ill., a corporation of Delaware Application November 21, 1945, Serial No. 630,063

14 Claims. 1 i

'The presentv invention relates tol ltelephone systems and, more particularly, to improved supervisory signal apparatus of the character disclosed and claimed in Patent No. 2,279,448; granted April 14, 1942; and issued to Thomas F. Crocker.

'A telephone system of the character disclosed in the Crocker patent mentioned comprises a switchboard provided with a number of operator positions and a number of groups of switching links individually associated with the operator positions, each operator position comprising equipment operative to select for use any idle one of the switching links in the associated group. Each switching link includes an answer switch and a call switch, theA answer switch of a selected switching link being operative to select a calling toll or trunk link accessible thereto, and the call switch of a selected switching link being operative to select a called toll or trunk link accessible thereto. The operation of the answer switch is controlled automatically by a distributor commonly associated with the groups of switching links, while the operation of the call switch is under the control of the lposition equipment at the lassociatedA operator position.

In a telephone system of this character, an operator I,frequently disconnects a switching link in Ause from the position equipment at the asso ciated operator position ,after the operation of the call switch thereof has been determined but prior to the complete operation of the call switch, particularly when .the switching. link is utilized to extend a connection to a local subscriber substation. In order to signal the operator in the event that the called local subscriber substation isfound to be Vbusy incident to the .complete operation of the local switch train, improved switching apparatus, including an electronic tone detector, is disclosed in the aforementioned Crocker patent to translate the busy tone signal returned from the local switch train into `a visual busy signal at the operator position associated with the switching link over which the call is extended. f

In this prior system the electronic tone detectors are employed only on local calls since all outgoing trunks terminate in operator positions located in rthe l,distant oiiices, calls to subscribers in these oilces being completed by the operators-fthereat. When the `outgoing trunks terminate in lincoming selectors at the distant offices-so that calls to subscribers located therein maybe completed by the originating operator, it is desirable that the originatingloper-ator be endistant offices.

abled to disconnect the switching link in use from the position equipment prior to the completion of the connection, in the same manner as for calls extended to local subscribers, in order to free the operator for a new call. In order to provide supervision to the operator under these conditions it has heretofore been necessary to either employ a special toll switch train in the distant offices which returns a ash busy indication over the trunk to the switching link, if the called line is in use, in place of the usual busy tone signal, or to rely on the electronic tone detector associated with the switching link to convert the busy tone signal returned over the trunk by a local switch train in the distant office into a visual busy signal at the operator position associated with said switching link. There are many instances when it is uneconomical, or otherwise undesirable, to employ toll switch trains in the While it is theoretically possible to employ local switch trains in these offices and rely on the tone detectors in the originating office to convert the busy tone signals into visual signals, such operation is not entirely satisfactory due to variations in the level of the busy tone returned from different distant ofces and due to the presence of inductive disturbances on the trunks which may, vfor example, be produced by neighboring power lines.

It is accordingly an object of the present invention to provide an improved switching apparatus, which may be located in the terminating oiice of an inter-office connection, and which is operative to repeat audio frequency supervisory signals transmitted thereto from a local switch train in the terminating oce as a characteristic direct current signal over the interofce trunk, wherebyan operator located in the originating cnice receives the same supervision as would normally be obtained from a toll switch train in `the terminating olce.

A further object of the invention is to provide an improved switching apparatus in accordance with `the preceding object wherein a small numn ber of electronic tone detectors are enabled to serve a large number of inter-ofce trunks.

Another object of the invention is to provide a switching apparatus of the character described in which tone detectors commonly associated with a group of trunk circuits include provisions for effecting their release vfrom a trunk circuit in use -in response to either a busy ltone signal, a ring back tone signal, or the lapse of a predetermined time interval following the transmission of a digit.

Fig. 3 is a schematic diagram of a trunk circuit and associated tone detector whichmay be employed in the system shown in Fig. l,

Fig. 4 is a schematic diagram of a-trunk circuit which may be employed in the system shown in Fig. 2, and y Fig. 5 is a schematic diagram of one of a group of tone detectors which may be commonly associated with a group of trunk circuits such as that shown in Fig. 4.

, Briefly considered the` invention comprises a trunk circuit which is connected between the terminating end of an inter-cnice trunk and an incoming selector. A plurality of electronic tone detectors; which are operative to convert audio frequency supervisory signals,A such as busy tone, into direct current signals suitable for relay operation; are either individually associated with corresponding ones of a group of the trunk circuits, or are commonly associated with the group of trunk circuits. In thelatter casemeans are provided for connecting an idle one of the tone detectors to a trunk circuit in response to the seizure thereof by a distant operator. The trunk. circuit is arranged to repeat call directing impulses irom the trunk to the incoming selector and succeeding switches in the usual manner. A conversation channel is not provided until after a connection has been established in order to prevent audio frequency supervisory signals from being returned over the trunk while a connection is being set up. Such signals may otherwise interfere with voice frequency dialling repeaters which may be used in a preceding link of the connection. The tone detector associated with a trunk circuit is rendered operative to test for the presence of busy tone after each seriesof call directing impulses has been repeated by the trunk circuit. A test after each series of impulses is required since the number of such series re` quired to complete a connection may not be fixed. In the event that a busy condition is encountered, the tone detector causes the polarity of the potential normally applied to the interofce trunk conductors to be intermittently reversed, thereby causing a visual :busy indication to be given an operator at the originating oflice in the usual manner. In each of the two forms of the invention illustrated, means are provided for establishing a conversation circuit either in response to the answering of a call, or in response to the elapse of a predetermined time intervalfollowing the transmission of a series of impulses to the incoming selector orv succeeding switches in the local switch train. In the modication employing a common group ci tone detectors the tone detector connected to a trunk circuit in use is released when the conversation circuit is established. In this modication the .tone detector is also operative to establish a conversation circuit and effect its release from the trunk circuit in response to the return of ring back tone over the local switch train,fwhereby thelength of time during which a tone detector mustremain connected with a trunk circuit in use is reduced to a minimum.

Referring now to the drawings, in Figs. 1 and 2 there is shown a subscriber substation I, located in ofce A, having access to an operator position 2 via a lineswitch and selector. A call initiated by a calling party at the substation I may be extended by an operator at the operation position v2 'through an outgoing trunk circuit 3, over an inter-cnice trunk 4, through an incoming trunk circuit 5 or 5', and over a local switch train to a subscriber substation 6. The incoming circuit 5 has a tone detector 1 individually associated therewith and the incoming trunk circuit 5has-a group of tone detectors 1 commonly associatedI therewith. The trunk circuit 5 and associated tone detector 1 are shown indetail in Fig. 3.' The trunk circuit 5 is shown in detail y in Fig. 4A and one of the tone detectors included in the group 1 thereof is shown in Fig. 5. The remaining equipment may follow conventional practice and, since the detailsof operation thereof are not essential to a complete understanding of the invention, have not been shown in detail.

Referring now speciiically to Fig. 3, the trunk circuit illustrated therein is seized from a distant oiiice bythe closure of an obvious loop circuit to relaySI over` the inter-office trunk conductors 315and 316. Relay 3I9 operates; closes a loop circuit to an associated incoming selector over conductors 311`and 318 through contacts 39| and BI I, the upper winding of relay 3I5, and contacts 326 and 3D2; and closesV a circuit to relay 320 at contacts 3I2. Relay 32o operates and closes the Time Pulse I lead, TPI, to the upper winding of relay 310 at contacts 322. The incoming selector returns ground over the control conductor 319, in response to the closure of a loop circuit thereto, thus completing a circuit to the lower winding of relay 3I5. The two windings of relay 3I5 act in opposition at this time.

A number of series of impulses are now transmitted over the trunk conductors 315 and 316 from the distant cnice to direct the operation of the local switch train,'including the incoming selector, in the usual manner. Relay 3I0 repeats the impulses received over the trunk to the local switch train over conductors 311 and 318 at contacts 3l I. Relay 3IIl also closes an intermittent circuit to relayv 325 through contacts 32I at contacts 312 during each series of impulses. Relays 320 and 325 are provided with copper sleeves which render them slow to release and consequently enable them to remain operated during impulsing'.

During the transmission of each series of impulses, relay 325 operates, cuts resistance 330 in series with the upper winding of relay 3I5 at contacts 326c`onnects resistance 38I in parallel with this series 'combination at contacts 321, and closes a circuit to relay 330 at contacts 329 from ground through contacts 323; Relay 330 operates and closes a holdingcircuit to itself in series with relay 335 and contacts 34I at contacts 33I.

Relay 325 restores shortly after the termination ofr each series of impulses and opens the operating circuit to relay 330 at contacts 329, thereby removing a short circuit from relay 335. The latter relay operatesin series with relay 330 and closes an intermittent circuit to slow-to-operate relay 340 through the weighted spring contacts 336 and contacts 323. After several seconds the, amplitude of vibration of the weighted spring contacts 336is reduced suiiiciently to enable re-` lay 340 to operateandopen the circuit to relays .7-, ahorras consequently restore. The restoration of relay 335 permits relay 348 torestore. I

The primary. windingof 'transformer 388 is normally bridgedvacross conductors`311 and 318 in multiple With the upper winding of relay 3|5 and inseries withcondenser 389 while the trunk circuit is in use. Relay 325 disconnects transformer 388 fromthe local line atcontacts 321 and 328 during impulsing. iThe secondary winding of transformer 388 is connected in the input circuit of the rst stagejof an'electronic tone detector comprisingvacuumtubes 382, 383 and 384, and their associated circuits. The first two stages of the tone detector operate as Class A amplifiers and are` tuned tothe frequency of the lousyv tone signalwhich is attimes transmitted over'conductors 311 `and318 by the local switch train. The last stage of the tone detector operates as a plate detector. It may be noted that the grid bias for the detector tube 384 is conveniently obtained from 'the exchange battery, the positive terminal of which is connected to ground. The tubes 382 and 383 may be the two triode sections of a type |2SN1 tube and tube 384 may be a type 25L6 beam power amplifier tube. The heaters of these tubes are energized from the exchange battery in series with relay 365. The latter relay is normally operated and connects the positive terminal ofv a common B supply to the anodes of tubes 382 to 384 through resistor 385. The negative terminal of the B supply is connected to ground. vResistor 385 and condenser 386 constitute a decoupling filter which prevents interaction between different tone Vdetectors energized from the same B supply. In the event that the heater element of one of the tubes burns out, relay 365 restores, closes an alarm circuit at contacts 361, and disconnectsv the -I-B supply lead from the tone detector and transfers it to resistor 381 to maintain the loady on the common B supply constant.

In the event that busy tone is returned over conductors 311 and 318,` after one or more series of impulses have been transmitted, either in response to an all-trunks-busy or a called line busy condition, relay 368 will be operated during the receipt of each pulse of tone. lThe busy tone is usually interrupted at a rate between 60 and 120 times per minute. Therefore relay 368 will be operated two or more times during the time interval which elapses between restoration of relay 325 and the operation of relay 348 following the termination of a series of impulses. As relays 338 and 335 are operated during this interval, the rst operation of relay 368 closes a circuit from ground through vcontacts 323, 36|, 348, 331 and 35| to relay 355. Relay 355 operates and closes a holding circuit to itself in series with relay 358. When relay 368 restores it opens the operating circuit to relay 355 and removes a short circuit from relay 358 at contacts 36|. Relay 358 roperates in seriesv with relay 355 and prepares lay 3|8 at contacts 386 and 381. Relay 385i follows7 the succeeding -operations of relay '368 tope'riod-- Relay 345v ically reverse the polarity of conductors 315 and;l 316`thereby causing.V the' operator in theV distant',

ofiice to receive a ash-busyindication in the same manner as usually employed in connection with toll 'switch trains. The operator will then release the connection by opening the circuit between the trunk conductors, thereby releasingv the called line, without encountering an alle.

trunks-busy condition, and it is found to be idle; ring back tone will 'be returned over conductors 311 and 318 from theconnector switch of the local switch train. The tuned circuits employed.

in the tone detector ordinarily prevent relay 368 from being operated in response to ring back tone,

but the timingsystem comprising relays 335 toV 355 is a further precaution to guard against false operation of relay`385 in response to the ring back tone. This timing system requires that relay 388 be operated at least twice, and each operation must be of sufficient `duration to enable operation of one of the slow-to-operate relays 345 and 355, within a few seconds. Since ring back tone signals are usually pulsesof one second duration spaced at ve second intervals, relay 368., could not beoperated twice within the time inter,

val measured by relays 335 and 348 even though it did respond to the pulses of ring back tone.

When the called party answers the direction of current iiow over conductors 311 and 318 is reversed by the connector switch ofthe local switch train in the usual manner. As the two windings of relay SI5 then act in aiding relationship, this relay operates and closes ground extended over conductor 319 to the upper winding of relay 388 at contacts 3|8. Relay 388 operates, switches the trunk conductors 315 and 318 through to conductors 311 and 318 free of all attachments at contacts 38| and'382, and closes a multiple circuit to its upper winding at contacts 383.` All operated relays, excepting relays 388 and 365, are now released in response to the restoration of relay 3|8.

The trunk circuit may `also be switchedr through at the end of a predetermined time in`v terval even though no answer signal is received from the called party. If the call was extended to an operator position in the terminating ollce, for example, no reverse battery signal would be received upon answering of the call by an operator thereat. The timing function is accomplished by grounded by the timer, not shown, the upper n winding of relay 318 is energized, causing it to operate and close a locking circuit to its lower winding from ground through contacts 323 33|, 341, and 312. When relay 338 operates in response t0 the receipt of a series of impulses it opens the locking circuit to relay 318 atcontacts 33| causing`A it to restore. This cycle of operations is repeatedy for eac-h received series of impulses Yuntil the last series has been transmitted, after which relay `338l l isfno longer operated. Consequently, the lower winding ofrelay 390 will be` energized through contacts 31|' fromthe momentary ground pulse transmitted over lead TP2 by the timer. Relay 330 operates, vswitches the trunk through at contacts' 30| and 302, lvand closes a locking circuit to its Aupper winding at contacts 303to the ground extended over conductor 319 by the local switch train. The spacing between" the ground pulses which (are transmitted over conductors TPI and TP2 by the timer is longer than the normal spacingbetween successive series of impulses which are received over the trunk, whereby switching through will not occur until after the last series of impulses has been received. If a busy condition is encountered, as previously described, the operation of relay 345 opens the operating and locking Acircuits to relay 310 at contacts 346 and 34,1 to prevent lthe subsequent operation of relay 300.

Referring now specically to Figs. 4 and 5, the trunk circuit illustrated therein is seized from a distant office by the closure of an obvious loop circuit to relay 439 over the inter-office trunk conductors 499 and 49|. Relay 43D Aoperates and closes a circuit to relay 440 at contacts 432. Relay 44D operates; closes a loop circuit to the incoming selector over conductors 492 and 493 through contacts 4I I and 43|, the' upper winding of relay 460, and contacts 44| and 412; closes a circuit to the lower winding of relay 450 at contacts 442; closes a circuit from wiper 484 through contacts 414, 444, 416 and 485, rotary switch magnet 480, and to battery through contacts, such as 515 and '595, of idle ones of the tone detectors in the common group thereof; and closes a circuit t relay 413 in series with magnet 480 from ground through contacts 445, 4|4 and 499. If the tone detector, which is connected to the contacts upon which wipers 48| and 485 are standing, is busy or out of service,` wiper 484 will be grounded thereby short-circuting relay 41|) and causing magnet 480 to operate self-interrupting to drive the wipers 48| to 495 until they stand on contacts connected to an idle tone detector. In the case of passing over contacts connected to tone detectors in use, the operatingground for magnet 480 is obtained from operated contacts, such as 414, and a wiper, such as 434, of the trunk circuit with which the tone detector is being used. In the case of passing over contacts connected to tone detectors whichlare out of service, the operating ground for magnet 48D is obtained from restored contacts, such as 513, of such tone detectors.

Assuming that the tone detector illustrated is thev rst idle one of the group reached by the rotary switch wipers 48| to 485, the short circuit will be removed from relay 410 permitting it to operate in series with magnet 489 when the rotary switch wipers stand on the contacts connected to the illustrated tone detector. The resistance of relay 41D is too high to permit magnet 480 to operate in Vseries therewith. When relay 419 operates it prepares circuits between the trunk circuit of Fig. 4 and the selected tone detector shown in Fig. 5 through wipers 48|, 48,2, 483 and 485 atI contacts 41|, 412, 413 and 415, respectively; closes ground through contacts 414, wiper 484 and contacts 513 to the upper winding of relay 580 and relay 59|) in series; and substitutes direct battery for battery through magnet 430 at contacts 416. l traced circuit. Relay 590 operates shortly thereafter, being rendered slow-toeoperate by means oa copper slug on the armature end of its core, closes groundthroughcontacts 56| and '58| in Relay 589 operates over the abovel ingjof A,rela'tyf551l' at'ciitacts 594. Relay 539 op-l crates, Ycloses a circuit tov relay 500 through'com" tactsrf59l 'and 53|,Iand'c1oses an inermittent circuit to slcwfto-foperaterelay 54|] through the weighted spring contacts"532. Relay 525 operates over a previously traced circuit. After a delay of several seconds relay 549 operates and opens' the series circuit'jto relays' 530 and 58D causing them to restore and release relays 539 andl 540. Duringjthe' time interval' measured by relays 530 and 54B a testfor the presence of busy tone on the local' line` conductors 492 and 493 will be made by the tone 'detector in the'manner subsequently described.' Thereason yfor making such a test immediately 'uponfseiz'ure of the tone detector is thatione'o'r more series of impulses may be transmitted from the circuit to the local switch train before the .tone detector is connected thereto,'particularly`wher`1 ther'efis no idle tone detector available at the timejth trunk'circuit is seized.

Series of impulses received over the trunk conductors 490 and 49|' are repeated to the incoming selector, andthe succeeding switches in the local 'switch train, by contacts 43| of relay 430 in theus'ualmanner.` Relay 450 operates, from ground through contacts 432 and 443, during the transmission of each series of impulses, discon` nects y'the 'local vline from the input circuit of the tone detector at contacts'45| and 453, cuts resistance 495 in series with the upper winding'o Arelay 460 at contacts- 452, shunts the preceding series combination byresistance 495 at contacts 45 l, and closes a circuit vfrom ground through contacts 454 andv 415 and throughi wiper 485 to the lower winding of relay 580..'v Relay 583 operates and closes its holding circuit through relay 530 at contacts 582.y When relay 455 restores upon the termination of yeach series vof impulses; it connects ,the input conductor 596 of the tone detector throughl wipery43| and contacts 41|, 45|, 43| and 4| I lto conductor 492; connects the input conductor 591 of the tone detector through Wiper 482 and ycontacts 412,453 `and`4|2 to conductor 493; and removes a short circuit from relay 530 at contacts 454. Relay 530 operates causing relay 590 to operate immediately and relay 54|) to operate vafter'several seconds delay as previously described. Upon thecperation of relay 549, relays 530, 589, 5|l andf54t'lV are released in turn.

if a busy tone .signal is returned over conductors 492 and ,493 it Willybeimpressed on the. primary winding'of transformer 5'3 over conductors 505 and 501 and through the blocking condensers in series` therewith. The impedance 509 is bridged across the. inputconductors through rthe blocking condensers to prevent-the introduction of surges in the input circuit ofthe `tone detector at the time that relay 459restcres and connects conductors 596'and'591 ,to conductors 492 and 493. The busy tone signalinduced in the secondary winding of` transformer 598 is impressed on potentiometers 5|2 a'nd`5l3 in series. As relay 500 is operated at this time the control grid of tube 5|4 is connected tov the, adjustable tap on potentiometer 512 throu'gl'iI contacts 50|, thereby impressing a relatively large portion, of theavailable signal voltage'on the inputcircuitof tube 5l4. The tone detectorA comprising tubes`5i4, 5I1 and 5|8 and their associated `cir'cuitsis ,Substantially identical to` that #cumuli/f desrg .ed shown 1.11 Fig-.3,

'contacts 591,56I, 504 and 5II.

4ground through contacts 593, 582, 525and 5I6,

except thatfthe tuned :circuits included therein are effective only when relay 500 is operated. The busy tone signal is ampliedbytubes 5I4 and 5I'I and is detected by tube 5I9 to cause relay 550 to operate in response to each tone r pulse. When relay 560 operates the rst time it 'closes a circuit to relay 520 from ground through Relay 520 operates and closes its holding circuit through contacts 52I, relay 5I0, and contacts 595 and 59h40 Whenrelay 560 restores it removes a short c ircuit from relay 5I0 at contacts 59|.

Relay 5H)v operates and prepares a circuit to relay 5I5 at contacts 5I I. The second operation of relay 569 `causes relay 5I5 to operate over an obvious cir- 15 resistance 5I9. Relay 409 does operate, howeven, 20 and closes a locking circuit to its upper windingr through contacts 402, 46| and 4I3 to the ground returned over the control conductor 494 of the local switch train, closes an interrupted ground from lead IG to relay 420 at contacts 403, and

opens the circuit to relay 410 at contacts 409. Relay 420 operates intermittently from the intern rupted ground on lead IG and periodically re- .verses the polarity of the potential impressed between conducto-rs 490 and 49I at contacts 42I and 30 422. This reverse battery signal causes a busy indication to be given the distant operator in the same manner as effected over a toll switch train. Relay 410 restores and disconnectsthe trunk circuit from the tone detector, opening the circuit to relay 590 at contacts 414. Relay 599 restores, opens circuits to relays 500, 5I0,5I5, 529, 525 and 549 at contacts 59I, opens circuits to relays 530 and 590 at contacts 593, and disconnects conductor TPI from relay 550 at contacts 594. The

tone detector is thus restored to normal and is free to function with another ltrunk circuit. When the distant operator releases lthe trunk in response to receipt of a busy indication, relay 430 restores, opens the loop circuit to the local switch train at contacts 43I, and opens the circuit to relay 449 at contacts 432. Relay 440 restores. When the local switch train is released, in response to the opening of the loop circuit thereto,

it removes ground from conductor 494 thereby releasing relay 400.

The tone detector repeats the above described test for busy tone upon the termination of each series of impulses transmitted over the trunk circuit. If a connection is completed to the called line land it is found to be idle, ring back tone will be returned over conductors 492 and 493upon the termination of the last series of impulses. The tone detector functions as previously described to test for busy tone upon the termination of this last series of impulses. When relay 500 restores at the conclusion of the busy tone test it transfers the control grid of tube 5I4 from potentiometer 5I2 to potentiometer 5I3 at contacts 50|, thereby lowering 65 the gain of the tone detector, and effectively removes the tuned circuits from the amplifier at contacts 502 and 503. With the tuned circuits of the amplier disabled, relay 560 is enabled to ,operate in response-to the ring back tonesignal impressed on the input of the tone detector.

.When relay-560 operates it opensthe circuit to relay ,525 at contacts 56|. Relay 525 restores after a brief delay and. closes a circuit. from 1ings of relays 400 and 4I0 in series.

'ings of relays 400 and 4I0 in series.

wiper 583, and contacts 413 to the lower wind- Both of these relays operate in this instance but the operation of relay 400 is merely incidental. p Relay 4I0 switches the trunk through clear of attachments at contacts 4II and 4I2, locks to ground on conductor 494 at contacts 4l3, and opens the circuit to relay 410 at contacts 4I4. Relays 410, 400, 430 and 440 restore in turn. When relay 410 restores it disconnects the trunk circuit from the tone detector and opens the circuit to relay 590 at contacts 414 to free the tone detector for another call, as previously described.

In the event that all tone detectors are 4busy and none is freed for use with a particular trunk circuit during the time that a connection is establishedy thereover,l somev provision must be made to immediately switch the trunk through when the called party answers. At the time that the called party answers the connector switch of' the local switch train reverses the direction of current now over conductors 492 and 493, thereby causing relay 490 to operate and complete a circuit from grounded conductor 494 through contacts '4I3, 46| and 40| jto the upper'winding of relay 4I0. Relay lIII) opcrates to switch the trunk through clear of attachments as previously described.

In the event that a tone detector is connected to a trunk circuit over which a connection to a vacant terminal is inadvertently established, neither ring back or busy tone will be received over conductors 492 and 493 to operate the tone detector. In order to free the tone detector for another call in such a case, a timing arrangement is provided therein which causes the trunk circuit to'switch through and free the tone detector connected thereto if no tone signal is received `within a' predetermined time interval following the transmission of a digit.v Thus when the tone detector is seized relay '599 operates and connects conductor TPI to the lower winding of relay 550 at contacts 594. Conductors 'IPI and TP2 are successively momentarily grounded a predetermined interval by a commontimer, not shown. `When conductor TPI is grounded relay 550 operates; closes a locking circuit to its upper winding from ground through contacts 593,` 582 and 552; and connects conductor TP2 through contacts 55I,'526 and 5I6, wiper 483, and contacts 413 to the lower wind- Each time that relay 580 operates in response to the transmission of a digit it opens the locking circuit to relay 550, if operated, at contacts 592. After the last digit has been transmitted and a busy tone test has been completed, relay 580 restores 'as previously described. Since this relay is not again operated relayy 550 will remain operated until conductor TP2 is ymomentarily Agrounded causing-relay 4I0 to operate over a previously traced circuit: Relay Y4II) switches the trunk through and frees the tone detector inthe same manner as described in response to the receipt of ring back tone. i

I While .particular embodiments of the -invention have vbeen illustrated, it is .to be vunderstood that numerous modifications may be incorporated therein without departing from the true spirit and yscopeof the, invention as defined in the subjoined claims.

What is claimed is: ,p

1. In a telephone system, anrautomatic switch train, a rst lineovyer which said switch train is 7,5 directively controlledftolextend connections to vama-'iss .frequency as a supervisory signal over said rst Yline in response to a particular condition, and

' means responsivefonly4 to Asaid audio frequency current for transmiti-,ingy a correspondingfdirect current signal over said second line.

2. In a telephone system, an automatic switch train, a first line over which said switch train is directively controlled, to extend connections to .calledi subscribers lines,l a second line over which-'digital impulses are at times received for .'difrecting` the operation' ofl said switch train,

means responsive to the digital impulses re-= ceived over-said second line for repeating corresponding impulses to said first line, said switch train operatedA to transmit an interrupted audio frequency current as a busy tone signal over said iirst line when the called subscribers line is in use, and means responsive to said audio frequency current for transmitting a corresponding direct current signal over said second line.

3. In a telephone system, an automatic switch train, a line' over which said switch train is directively controlled by digital impulses to extendy connections to called subscribers, means operated to transmit impulses representing digits over said line, saidswitch train at times operated to return current of an audio frequency as a supervisory signal over said line in response 'to a particular condition,l testing means operated in response to each transmission 'of impulses representing a digit over said line to test for the return of said audio frequency current thereover, said last meansr further operated to provide a direct-current indication in the event that said audio frequency current is returned during a test therefor. 1

4. In a telephone system, anautomatic switch train, a; line overl which said switch train is directively controlled by "digital impulses to extend connections to called subscribers, means operated to transmit digital impulses over said line, said switch train at times operated to re- Aturn currents of different audio frequency asv supervisory signals over said line in response to particular conditions,` testing means operated subsequent to the` transmission of digital impulses over said line toy successively test for the return of said lsupervisory signals thereover, said testing means further 'operated to effect one of a pluralityV of distinctive switching operations in .accordance with'the particular supervisory sig- `nal returned during the test.

, v5. Inria telephone system, ,an vautomatic switch trainfa first line overwhich lsaid switch train is directively controlled to extendconnections to called subscribers lines, a second line over which digital impulses are' at times received for directing the operation of said switch train, means re- 'sponsiveto the digital impulses received over said second line for repeating corresponding impulses to said rsty line, said switch train operated to 4return an interrupted audio frequency current incoming and outgoing lines connected to eachfof said repeaters, a group of tone detectors commonly associated with said repeaters, means re- 'sponsive to the seizure of one of said repeaters over the incoming line connected thereto for connecting an idle one of said tone detectors to said one repeater, said one tone detector operative, when so connected, to cause a characteristic direct current supervisory signal. to be trans'- mitted over the incoming line connected to said one repeater in response to the receipt of an audio frequency supervisory signal over the outgoing line connected thereto.

7. In a telephone system as claimed in lclaim 6, said rst means responsive to the detection of an audio frequency supervisory signal by said one tone detector to disconnect it from said one repeater.

8. In a telephone system, a group of repeaters, incoming and outgoing lines connected to each of said repeaters, a'group of tone detectorsA commonly associated with said repeaters, means responsive to the seizure of one of said repeaters over the incoming line connected thereto for connecting an idle one of said tone detectors'to said one repeater, said one tone detector operative, when so connected, to cause a characteristic direct current signal to be transmitted over the incoming line connected to said one repeater in response to the receipt of one audio frequency supervisory signal over the outgoing line connected thereto, or to cause said one repeater to switch through in response to the receipt of a different audio frequency supervisory signal over the outgoing line.

9. In a telephone system, a line over which interrupted tone signals having different rates of interruption are at times transmitted, a tone detector including a thermionic amplifier responsive to tone signals impressed on the input thereof to operate a relay in the output thereof, a plurality of timing means each corresponding to one of said interrupted tone signals, means for at times coupling the input of said tone detector to said line and for successively associating each of said timing means with the tone detector relay, each of said timing means operated to perform a distinctive switching operation in the event that theI corresponding tone signal is present on said line.

l0. In a telephone system as claimed in claim 9 wherein the interrupted tone signals also have diierent frequency characteristics, means operated to vary the frequency response characteristics of said tone detector as diierent ones of said timing means are associated withthe tone detector relay., y V

11. In a telephone system as claimed in claim 9 wherein the interrupted tone signals valso have different amplitudes, means operated to vary the sensitivity of said tone detector as diierent ones of said timing means are associated with the tone detector relay.

12. In a telephone system, a trunk line having direct current flowing therein, an automatic switch train operatedv at times by impulses over said line to extend a connection. to a called line, means in the switch train for transmitting a current of a particular frequency as a busy tone back to the line under certain conditions and for transmitting a current of a different Afrequency back to the line under other conditions, a tone detector associated with the trunk linejresponsive to current of said particular frequency, Aand circuit and relay means' operated by the respense of said detector for altering the direct current owng in said trunk line to signal thereover, said tone detector responsive to said different frequency at another time, and relay means responsive to the last mentioned response of said tone detector to perform a switching operation.

l13. A telephone system such as claimed in claim 12 in which there is means in the tone detector operated at a certain stage in the extension of the connection to vary the circuits of the tone detector to cause it to respond to sai different frequency.

14. In a telephone system, an automatic switch train, a trunk line over which said switch train is directively controlled by digital impulses from a calling party to extend connections to a called line, means controlled responsive to the receipt of digital impulses from a calling party to operate said switch train for completing a connection with a called line over said trunk line, means in said switch train controlled by said called line when busy for transmitting a current of a rst audio signal as a busy tone over said trunk line, means in said switch train controlled by said called line when idle for transmitting a current of a second audio signal as a ring back tone oversaid trunk line, a tone detector associated with said trunk line responsive to the currents of said rst and second audio signals, a relay operated by said tone detector responsive to the current of said rst audio signal for reversing battery over said trunk line, and another relay operated by said tone detector responsive to the current of said second audio signal for switching said trunk line to complete connections to said called line.

IMRE MOLNAR.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,654,068 Blattner Dec. 27, 1927 1,677,876 Wicks July 17, 1928 1,829,071 Stazak Oct. 27, 1931 2,355,642 Gose Aug. 15, 1944 

